(1) Field of the Invention
This invention relates to a biomagnetism measuring method and apparatus for determining bioelectric currents occurring in a site of interest in the body of an examinee or patient.
(2) Description of the Related Art
A living body produces minute biomagnetism as a result of bioelectric currents occurring in the living body. For example, the biomagnetism generated by bioelectric currents in the brain is called magnetoencephalo which includes induced magnetoencephalo formed by a stimulation applied to the living body, and spontaneous magnetoencephalo produced spontaneously by the brain such as alpha wave or spike wave associated with epilepsy.
In recent years, a multichannel SQUID sensor has been developed which uses SQUIDs (Superconducting Quantum Interface Devices) as magnetic sensors for measuring minute biomagnetism in the living body. The multichannel SQUID sensor has a multiplicity of magnetic sensors immersed in a coolant such as liquid nitrogen within a vessel called a Dewar. The magnetic sensors have coil portions for measuring magnetic fields, and SQUID sensor portions for converting the magnetic fields detected to voltages.
The multichannel SQUID sensor is placed outside a site of interest, e.g. the head, of a patient. The magnetic sensors in the multichannel SQUID sensor can carry out a noninvasive measurement of minute biomagnetism produced by bioelectric currents occurring in the brain. From biomagnetism information thus obtained, one can grasp locations, orientations and other states of the bioelectric currents occurring in the site of interest of the patient.
Conventionally, bioelectric currents formed in a patient""s site of interest are derived as follows.
First, a positional relationship between the multichannel SQUID sensor and patient""s site of interest is determined. There are various techniques for determining the positional relationship as disclosed in Japanese Patent Publication (Unexamined) H5-237065 and Japanese Patent Publication (Unexamined) H6-788925, for example.
Next, weight coefficients are computed which will enhance biomagnetism information based on a minute magnetic field formed by a bioelectric current flowing in each particular position in the patient""s site of interest. These weight coefficients are computed for each magnetic sensor included in the multichannel SQUID sensor. A technique for computing the weight coefficients is described, for example, in S. E. Robinson and D. F. Rose, xe2x80x9cCurrent Source Image Estimation by Spatially Filtered MGExe2x80x9d, Biomagnetism: Clinical aspects (1992).
A computation of convolution is carried out of biomagnetism information acquired by measuring the biomagnetism from the patient""s site of interest, and the weight coefficients for enhancing only the biomagnetism from each particular position. As a result, only the biomagnetism generated from the bioelectric currents in the particular position in the site of interest is enhanced. In this way, bioelectric currents in given positions in the patient""s site of interest may be determined.
By determining the states of bioelectric currents for numerous positions in the patient""s site of interest, the states of bioelectric currents occurring in the entire site of interest may be grasped. These bioelectric currents may be super-posed on a tomographic image of the patient""s site of interest acquired with an MRI apparatus or the like, to grasp the states of bioelectric currents on the tomographic image of the site of interest.
However, in order to enhance the biomagnetism from the bioelectric current occurring in each particular position in the patient""s site of interest, according to the prior art, a weight coefficient must be computed for application to each transfer constant, which will equalize the bioelectric current expressed by delta function set to the particular position in the site of interest, to a sum total of transfer constants with which the magnetic field is transmitted from the particular position to the respective magnetic sensors. This weight coefficient is computed from an inverse of a matrix based on a transfer constant for each magnetic sensor, and virtual biomagnetism information measured virtually by each magnetic sensor when the bioelectric current expressed by delta function is set to the particular position. However, when the solution of the matrix based on the transfer constant is very close to zero, the solution of the inverse matrix becomes enormous. With the weight coefficients computed from this inverse matrix and applied to the measured biomagnetism information, even the noise included in the information is enhanced. It is therefore impossible to grasp accurately the bioelectric currents occurring in the particular positions.
This invention has been made having regard to the state of the art noted above, and its object is to provide a biomagnetism measuring method and apparatus for accurately determining bioelectric currents occurring in particular positions in a patient""s site of interest, without being affected by noise included in measured biomagnetism information.
This above object is fulfilled, according to this invention, by a biomagnetism measuring method for measuring, with a plurality of magnetic sensors arranged adjacent a site of interest of a patient, biomagnetism generated from bioelectric currents flowing in the site of interest, and determining the bioelectric currents based on measured biomagnetism information acquired through the measurement, the method comprising the steps of:
computing transfer constants indicating degrees by which the biomagnetism is transferred from each particular position in the site of interest to each of the magnetic sensors;
regulating a matrix of the transfer constants;
computing a weight coefficient for each of the magnetic sensors to enhance biomagnetism generated from the particular position, based on virtual biomagnetism information virtually measured by the magnetic sensors and an inverse matrix of the regulated matrix, when a known bioelectric current is set to the particular position;
collecting measured biomagnetism information acquired by measuring the biomagnetism generated from the site of interest with the magnetic sensors;
computing enhanced biomagnetism information having the biomagnetism generated from the particular position enhanced by applying the weight coefficient to the measured biomagnetism information measured by each of the magnetic sensors; and
determining a bioelectric current in the particular position based on the enhanced biomagnetism information.
According to this invention set out above, a transfer function indicating a degree of transfer of biomagnetism in space is determined based on Biot-Savart""s law, for example. Transfer constants are obtained by substituting, for the transfer function, particular positions in the site of interest of the patient and positional relationships thereof with the respective magnetic sensors. A matrix formed of the transfer constants is regulated, whereby a solution of the matrix has a value remote from zero. This lessens the unstable condition of the matrix occurring when the solution of the matrix is very close to zero. Further, a weight coefficient is computed for each of the magnetic sensors to enhance biomagnetism generated from each particular position, based on virtual biomagnetism information virtually measured by the magnetic sensors and an inverse matrix of the regulated matrix, when a known bioelectric current is set to the particular position. These weight coefficients, which are based on an inverse matrix in a stabilized condition, enhance only the biomagnetism from the particular positions. Then, enhanced biomagnetism information is computed by applying the weight coefficients to the measured biomagnetism information obtained by measuring, with the magnetic sensors, the biomagnetism generated from the site of interest of the patient. The bioelectric currents in the particular positions are determined based on the enhanced biomagnetism information. Thus, this invention can avoid the inconvenience encountered in the prior art in which weight coefficients based on the inverse of a matrix whose solution is close to zero are applied to the measured biomagnetism information, thereby enhancing even noise included in the measured biomagnetism information. As a result, the invention enables a correct determination of the states of bioelectric currents, e.g. current sizes, directions and locations, in particular positions in a patient""s site of interest.
Preferably, the method according to this invention further comprises the step of outputting an arrow map showing the bioelectric current in each particular position in form of an arrow, the step of outputting an arrow map showing the bioelectric current in each particular position in form of an arrow, as superposed on a tomographic image including the particular position, the step of outputting an intensity distribution chart of the bioelectric currents generated in the site of interest, or the step of outputting a waveform chart of biomagnetism measured by each of the magnetic sensors. This method will then facilitate a visual recognition of the states of bioelectric currents in the particular positions in the patient""s site of interest or the states of biomagnetism measured. This feature is of advantage to diagnostic situations and the like.
It is preferred that the matrix regulating step is executed to add a predetermined value for a solution of the matrix of the transfer constants to be remote from zero. The predetermined value, preferably, is at least one of a unit matrix or a penalty matrix having a predetermined size. It is also preferred that the predetermined value is at least one of a unit matrix or a penalty matrix having a size corresponding to noise included in the measured biomagnetism information.
In another aspect of this invention, a biomagnetism measuring apparatus is provided for measuring, with a plurality of magnetic sensors arranged adjacent a site of interest of a patient, biomagnetism generated from bioelectric currents flowing in the site of interest, and determining the bioelectric currents based on measured biomagnetism information acquired through the measurement, the apparatus comprising:
a plurality of magnetic sensors arranged adjacent the site of interest;
a transfer constant computing device for computing transfer constants indicating degrees by which the biomagnetism is transferred from each particular position in the site of interest to each of the magnetic sensors;
a regulating device for regulating a matrix of the transfer constants;
a weight coefficient computing device for computing a weight coefficient for each of the magnetic sensors to enhance biomagnetism generated from the particular position, based on virtual biomagnetism information virtually measured by the magnetic sensors and an inverse matrix of the regulated matrix, when a known bioelectric current is set to the particular position;
a measured information collecting device for collecting measured biomagnetism information acquired by measuring the biomagnetism generated from the site of interest with the magnetic sensors;
an enhanced biomagnetism information computing device for computing enhanced biomagnetism information having the biomagnetism generated from the particular position enhanced by applying the weight coefficient to the measured biomagnetism information measured by each of the magnetic sensors; and
a bioelectric current computing device for determining a bioelectric current in the particular position based on the enhanced biomagnetism information.
With this apparatus, the weight coefficient computing device computes a weight coefficient for each of the magnetic sensors to enhance biomagnetism generated from the particular positions in the site of interest, based on an inverse matrix of the matrix regulated by the regulating device, and on virtual biomagnetism information. The bioelectric current computing device determines bioelectric currents in the particular positions based on the enhanced biomagnetism information obtained by applying the weight coefficients to the measured biomagnetism information.
Preferably, the apparatus according to this invention further comprises an output device for outputting an arrow map showing the bioelectric current in each particular position in form of an arrow, an arrow map showing the bioelectric current in each particular position in form of an arrow, as superposed on a tomographic image including the particular position, an intensity distribution chart of the bioelectric currents generated in the site of interest, or a waveform chart of biomagnetism measured by each of the magnetic sensors.
Preferably, each of the magnetic sensors employed in the apparatus according to this invention is a high-sensitivity magnetic sensor including, for example, a coil, and a super-conducting quantum interference device connected to the coil.
The regulating device of the above apparatus may, for example, add a predetermined value for a solution of the matrix of the transfer constants to be remote from zero. Preferably, the predetermined value is at least one of a unit matrix or a penalty matrix having a predetermined size. It is also preferred that the predetermined value is at least one of a unit matrix or a penalty matrix having a size corresponding to noise included in the measured biomagnetism information.
The apparatus according to this invention may further comprise a stimulating device for applying at least one of an electric stimulation, an acoustic stimulation and an optical stimulation.